Initial production rates on Valiant Petroleum's Causeway field in the North Sea have proved lower than expected, due to problems associated with the long horizontal nature of the well.Valiant, with a 64.5% stake, is the operator of the field while Canada's Antrim Energy owns the rest of the rights to the field.The Causeway field in UKCS Block 211/23d has started production with gross output rates of around 4,500 barrels of oil per day (bopd) on a 53% choke; a "choke" is a device installed to restrict the flow of fluids. Valiant is currently investigating a range of well stimulation and intervention options to assist in achieving anticipated production rates, if necessary.The next phase of work on Causeway and the Fionn oil field is anticipated to commence early in 2013 and to be completed by the middle of the year, the company revealed.Stephen Greer, Antrim President and Chief Executive Officer, said the production of oil from its UK North Sea interests represents a major milestone for Antrim."Causeway is expected to contribute substantial cash flow to Antrim, as we continue to develop our existing assets and explore other opportunities in the UK and elsewhere," Greer said.Valiant, meanwhile, said that with new Dons field wells coming on stream in the second half of 2012 and the addition of Causeway, it is anticipating a year-end exit rate in excess of 11,500 bopd. Due to start-up delays and production performance to date at Causeway, Valiant's production in 2012 is anticipated to average around 6,500 bopd.JH